IBM disputes Google claims of 'quantum supremacy'

IBM accuses Google of over-hyping its quantum computing claims

IBM has called into question the ‘quantum supremacy' claims made by Google last month.

The concept of quantum supremacy was developed by John Preskill, the Richard P. Feynman professor of theoretical physics at the California Institute of Technology, as a test that would indicate that quantum computers were finally able to outperform conventional computers.

A successful test of quantum supremacy would show that a quantum computer can run a complex calculation that a conventional computer cannot do within a reasonable length of time.

An ideal simulation of the same task can be performed on a classical system in 2.5 days and with far greater fidelity

However, in a blog posting, IBM quantum computer specialists Edwin Pednault, John Gunnels and Jay Gambetta suggest that the quantum supremacy calculations that Google claimed would take a conventional computer 10,000 years to compute should take just days, at most.

"Recent advances in quantum computing have resulted in two 53-qubit processors: one from our group in IBM and a device described in the leaked preprint from Google," they write.

They continue: "In the preprint, it is argued that their device reached ‘quantum supremacy' and that ‘a state-of-the-art supercomputer would require approximately 10,000 years to perform the equivalent task'.

The concept of 'quantum supremacy' showcases the resources unique to quantum computers

"We argue that an ideal simulation of the same task can be performed on a classical system in 2.5 days and with far greater fidelity. This is in fact a conservative, worst-case estimate, and we expect that with additional refinements the classical cost of the simulation can be further reduced.

"Because the original meaning of the term quantum supremacy, as proposed by John Preskill in 2012, was to describe the point where quantum computers can do things that classical computers can't, this threshold has not been met."

Classical computers have resources of their own such as a hierarchy of memories and high-precision computations in hardware

The IBM quantum computing specialists add that Google's claim that the calculations would take a conventional computer 10,000 years is based on the belief that the numbers involved would be too big to be fully stored in memory, therefore requiring time-consuming workarounds. However, they add, the Google engineers' assumptions don't take into account a number of advantages that a conventional computer would also bring to bear.

"The concept of 'quantum supremacy' showcases the resources unique to quantum computers, such as direct access to entanglement and superposition.

"However, classical computers have resources of their own such as a hierarchy of memories and high-precision computations in hardware, various software assets, and a vast knowledge base of algorithms, and it is important to leverage all such capabilities when comparing quantum to classical," they write.

"When their comparison to classical [computing] was made, they relied on an advanced simulation that leverages parallelism, fast and error-free computation, and large aggregate RAM, but failed to fully account for plentiful disk storage."

Their own simulation of the same calculations used a much wider range of features, including "circuit partitioning, tensor contraction deferral, gate aggregation and batching, careful orchestration of collective communication, and well-known optimization methods such as cache-blocking and double-buffering" in order to bring the time down from tens of thousands of years to just days.

"New and better classical hardware, code optimizations to more efficiently utilize the classical hardware, not to mention the potential of leveraging GPU-direct communications to run the kind of supremacy simulations of interest, could substantially accelerate our simulation," they claim, adding that the term ‘quantum supremacy' is not only much misunderstood, but also misleading.

In addition to claiming that researchers at the company had achieved quantum supremacy, Google also claimed last year to have developed a 72-qubit quantum processor, while IBM in 2017 revealed plans to start offering quantum computing ‘in the cloud'.